The invention relates to an apparatus for use in a sewage treatment plant.
Effluent flow entering a sewage treatment plant contains solid materials, such as rags, paper, polythene and other plastic sheeting, and the like, which cannot be processed by the treatment plant. Solids can be removed from the flow by screens or sieves which capture the solids. The screens or sieves are then periodically or continuously operated for cleaning to remove the captured solids, commonly referred to as screenings, for disposal. One such screening apparatus known in the art is the xe2x80x9ccontinuous-belt screenxe2x80x9d which includes a continuous driven belt screen presenting a continuous, moving screening area to the input sewage flow to effect removal of screenings within the sewage flow. The belt screen removes any solid materials carried by the input sewage flow whilst permitting the liquid phase of the sewage to flow onwards to the subsequent processing stages of the treatment plant.
In any sewage processing apparatus, it is desirable to reduce the head loss (i.e. the difference in sewage flow level between the flow upstream of the screening apparatus and the flow downstream of the screening apparatus) as this permits higher sewage flow rates to be screened with increased efficiency. The head loss is determined by the screening area which is presented to the input flow; the smaller the screening area presented to the input flow, the greater the head loss and the less efficient the apparatus at higher flow rates.
In conventional arrangements, as shown in FIG. 6, the continuous belt screen 6 is located in an inlet channel 10 which delivers the sewage flow 12 to the inlet stage, or inlet works, of the sewage treatment plant where the inlet sewage flow is screened. Following the inlet stage, screened sewage is passed through a grit removal facility, where grit and other small stones are removed from the sewage flow. The screened sewage is then delivered to the subsequent sewage processing stages. However, as the depth of the inlet channel 10 is limited, only a limited screening area is presented to the sewage flow 12 and the head loss, represented by D2, is significant. Such arrangements therefore tend to be inefficient, especially at higher flow rates.
In order to overcome this problem, several continuous belt screens may be accommodated in parallel within one or more inlet channels, thereby reducing the total head loss through the apparatus. However, as this solution requires many belt screens to be installed, the cost of the apparatus is increased. A greater installation space is also required for the apparatus. Alternatively, in some installations it is possible to provide a depression in the floor of the inlet channel, the belt screen being located in the depression so as to present a larger screening area to the input flow. However, it is not usually possible to provide depressions in many existing installations where belt screens are arranged in the inlet channel due to the location of the inlet channel at ground level. Additionally, providing such a depression in the inlet channel leads to a reduced flow velocity which can cause settlement of debris in front of the screening apparatus.
It is an object of the present invention to provide an apparatus for use in a sewage treatment plant which alleviates these disadvantages.
According to the present invention, there is provided an apparatus for use in a sewage treatment plant including a settlement tank, an inlet channel for delivering an input sewage flow to the settlement tank, and a screen arrangement having a continuous belt screen for capturing screenings contained within the sewage flow to enable their subsequent removal therefrom, wherein the screen arrangement is arranged within the settlement tank.
Conveniently, the apparatus may be used in an inlet stage, or inlet works, of a sewage treatment plant.
The apparatus provides the advantage that, as the settlement tank has a large volume and as the screen arrangement is arranged within the settlement tank, the screening area presented to the input sewage flow is greatly increased. Thus, the head loss can be reduced, or substantially eliminated, compared to known arrangements in which the screen arrangement is arranged within the inlet channel. In this way, the efficiency of the apparatus can be improved and the cost of installation can be reduced.
The settlement tank may comprise an inlet chamber, the input sewage flow being delivered to the inlet chamber by the inlet channel. The inlet chamber may deliver the sewage flow directly to the screen arrangement or may deliver the sewage flow to the screen arrangement through a transition channel. The transition channel or the inlet chamber may be provided with means for isolating the screen arrangement from the sewage flow, for example a stop-gate.
Conveniently, the floor of the inlet chamber is raised above the floor of the settlement tank. The floor of the inlet chamber may be provided with a recess or depression for retaining stones or rocks contained within the sewage flow prior to delivery of the flow to the screen arrangement. The inlet chamber may also be provided with means for removing stones or rocks retained within the recess or depression. Removal of stones or rocks from the sewage flow prevents damage being caused to the screen arrangement and any screenings washing apparatus located downstream of the screen arrangement.
Alternatively, if a transition channel is provided, the floor of the transition channel may be raised above the floor of the settlement tank, the recess or depression for retaining stones or rocks contained within the sewage flow being provided in the transition channel.
The apparatus may further comprise a removal apparatus for removing settled products, such as grit and small stones, from the settlement tank. The removal apparatus may be a pumping apparatus. The apparatus may also include a separation apparatus for separating grit and small stones removed from the settlement tank from other products, such as vegetable products and other relatively fine particles of solid sewage materials, which may settle in the settlement tank.
The apparatus may further comprise a washing apparatus for washing screenings removed from the sewage flow by the screen arrangement. The settlement tank may be provided with an outlet weir, over which screened, and preferably de-gritted, sewage is discharged to an outlet chamber, the walls of the outlet chamber and the settlement tank being formed from the same housing. The settlement tank and the outlet chamber may be covered. The washing apparatus and, if present, the separation apparatus may be supported on or above the covering. Thus, a reduced installation space is required to accommodate the complete apparatus.